For constructions such as a house or a building, there is a need for saving energy consumption through improvement in the heating or cooling efficiency. As the efficiency of heating or cooling depends on the heat-insulating performance or air-tightness of the construction, heat-insulating wall materials and heat-insulating window glasses have been developed.
However, a heat-insulating window glass generally has a higher heat-through ratio than a heat-insulating wall material, hence a lower insulating performance. Accordingly, in order to achieve energy consumption saving, it is necessary to enhance the heat-insulating performance of the heat-insulating window glass. As a heat-insulating window glass having a higher heat-insulating performance, a multiple glazing is known. This multiple glazing is shown in FIG. 5.
FIG. 5 shows a cross-sectional construction of a conventional heat-insulating multiple glazing. This heat-insulating multiple glazing 100 includes two sheets of sheet glasses 101, 102 overlapped with each other with spacers 103 interposed therebetween sealing the peripheries of the plates and also with dry air being charged in the intermediate gap. This heat-insulating multiple glazing may achieve a heat-insulating performance equivalent to a heat-through ratio of 3.0-4.0 kcal/m.sup.2 hr.degree. C.
On the other hand, the heat-insulating wall material provides a heat-through ratio of 1/10 (0.3-0.4 kcal/m.sup.2 hr.degree. C.) of that of the heat-insulating multiple glazing 100. Accordingly, enhancement of the heat-insulating performance of the heat-insulating multiple glazing may lead to energy consumption saving. Following measures (a)-(d) are known for enhancing the heat-insulating performance of the heat-insulating multiple glazing 100.
(a) Low-radiating films are formed on inner surfaces of the sheet glasses 101, 102 of the heat-insulating multiple glazing 100, so that the low-radiating films may reflect infrared beam, thereby improving the heat-insulating performance. PA1 (b) The dry air charged between the sheet glasses 101, 102 is replaced by rare gas so as to restrict convection between the sheet glasses 101, 102. As the rare gas, gas such as argon or krypton which hardly causes convection is employed, so that the convection between the sheet glasses 101, 102 may be appropriately restricted. PA1 (c) The heat-insulating performance may be enhanced by increasing the number of the sheet glasses 101, 102 of the heat-insulating multiple glazing 100 or by increasing the gap between the sheet glasses 101, 102. PA1 (i) The low-radiating film must be limited to those which have a radiating ratio of 0.15 or lower and whose heat-through ratio hardly drop any further from 1.0 kcal/m.sup.2 hr.degree. C. PA1 (ii) If a forcible attempt is made to improve the heat-insulating performance of the vacuum multiple glazing by means of a low-radiating film, there develops a temperature difference between the front side and the rear side of the sheet glass. And, if the difference is significant, this may lead to breakage.
The heat-insulating performance may be enhanced by depressurizing the gap between the sheet glasses 101, 102 of the heat-insulating multiple glazing 100 thus restricting air convection.
However, in the case of (a), it is necessary for the low-radiating film to have light-through property so as to allow introduction of light into the indoor space. Then, in order to satisfy both of the low-radiating property and the light-through property, it is necessary to restrict the heat-through ratio of the heat-insulating multiple glazing 100 to about 1.0-1.5 kcal/m.sup.2 hr.degree. C. Hence, this value is still insufficient when compared with the heat-through ratio: 0.3-0.4 kcal/m.sup.2 hr.degree. C. of the heat-insulating wall material.
In the case of (b), in combination with the low-radiating film, the heat-through ratio of the heat-insulating multiple glazing 100 may be reduced to 1.0 kcal/m.sup.2 hr .degree. C. However, this is still insufficient when compared with the heat-through property: 0.3-0.4 kcal/m.sup.2 hr.degree. C. of the heat-insulating wall material.
In the case of (c), the heat-through ratio of the heat-insulating multiple glazing 100 may be reduced to 0.5 kcal/m.sup.2 hr.degree. C. However, the increase in the number of the sheet glasses 101, 102 will result in increase in the thickness of the heat-insulating multiple glazing 100. Then, the cost of the window frame for use with the heat-insulating multiple glazing 100 will increase. Further, the increase in the number of sheet glasses 101, 102 will result also in the cost of the heat-insulating multiple glazing 100.
In the case of (d), the heat-through ratio of the heat-insulating multiple glazing 100 may be reduced to 1.0 kcal/m.sup.2 hr.degree. C. approximately. Accordingly, if the low-radiating films are formed on the heat-insulating multiple glazing 100, it may be possible to reduce sufficiently the heat-through ratio without inviting increase in the thickness of the heat-insulating multiple glazing 100.
However, in order to allow the heat-insulating multiple glazing 100 in a building construction, it is necessary to maintain the gap between the sheet glasses 101, 102 under the evacuated depressurized condition for an extended period of time; and the gap between the sheet glasses needs to be firmly sealed by a high-temperature treatment (above 400.degree. C.) like the welding.
Further, if the low-radiating film is formed on the surface of the heat-insulating multiple glazing, there is the risk of the low-radiating film being damaged. So, it is preferred that the low-radiating film be formed on the inside of the heat-insulating multiple glazing. Accordingly, it becomes necessary to form the low-radiating film before the sealing of the peripheries of the two sheet glasses.
Incidentally, most of such low-radiating films are vulnerable to high temperature, so that they cannot effectively resist the high temperature used in the welding treatment for sealing the heat-insulating multiple glazing 100. A low-radiating film capable of effectively resisting the high temperature is known which is formed of tin oxide doped with fluorine formed by the thermal decomposition method. This provides a radiation ratio of 0.15. Therefore, if this low-radiating film is formed on the heat-insulating multiple glazing, the heat-through ratio of the heat-insulating multiple glazing 100 will hardly be reduced further from 1.0 kcal/m.sup.2 hr.degree. C. Hence, the heat-through ratio of the heat-insulating multiple glazing 100 is still insufficient, when compared with the heat-through ratio of the heat-insulating wall material ranging between 0.3-0.4 kcal/m.sup.2 hr.degree. C.
Then, in view of the above-described problems of the prior art, an object of the present invention is to provide art capable of enhancing the heat-insulating performance without increasing the thickness of the heat-insulating multiple.